Traditionally, treated domestic wastewater is disinfected by the addition of chlorine. In recent years, many drinking water facilities have converted to chloramination to disinfect potable water. Chlorine reacts quickly with ammonia (present or added) and any organic nitrogen present in the water to form monochloramine, dichloramine (from ammonia) and organic chloramines (from organic nitrogen compounds). The relative amounts of mono-, di- and organic chloramines formed during the chloramination process depend on the ratio of chlorine-to-nitrogen, pH, temperature, mixing efficiency, and time of contact. Monochloramine and dichloramine (inorganic chloramines) are very effective biocides, but organic chloramines, as a class, have poor disinfection properties.
Monochloramine is the preferred disinfectant for most wastewater treatment facilities that employ biological-oxidation treatment processes (known as secondary treatment). Prior to disinfection, most secondary treatment plants will contain ammonia levels between 0.5-10 mg/L (as nitrogen, N). At pH 7 to 8, and when the mass ratio of chlorine to ammonia-nitrogen is 5:1 or less, all chlorine added is converted to monochloramine. When the applied chlorine (as Cl.sub.2) to ammonia-N ratio exceeds 5:1, dichloramine is formed with a corresponding drop in the total biocide concentration (monochloramine+dichloramine, expressed as Cl.sub.2). This phenomena is known as breakpoint chlorination and is depicted in FIG. 1.
Although a superior disinfectant, dichloramine formation is usually avoided since more chlorine is unnecessarily consumed and results in a corresponding decrease in total oxidant concentration. Also, the presence of dichloramine can lead to pungent odors in the chlorine contact chambers of some secondary treatment facilities. Dichloramine is not desirable in potable water since its presence can affect both taste and odor.
According to White, "Handbook of Chlorination", Van Nostrand/Reinhold, 3rd Ed., New York, pp. 589-606 (1993), secondary biological wastewater treatment can produce soluble organic nitrogen concentrations in the range of 3-15 mg/L (as N). White also states that if the mixing of chlorine (either gaseous or liquid soda bleach) with the wastewater is poor, the chlorinated species will tend to split between monochloramine and organic chloramines. Several studies have shown that organic chloramines have significantly less germicidal activity than monochloramine.
Other studies, Yoon & Jensen, Water Environ. Res., 67,842 (1995) and Isaac & Morris, Environ. Sci. Technol., 17, 739 (1983), have indicated that, with time, monochloramine can transfer its chlorine to nitrogenous organics, producing the weaker disinfecting organic chloramines. Thus, the germicidal efficiency of chlorinated wastewater has a tendency to decrease with time.
One way to ensure the adequacy of disinfection is to main a total oxidant residual. Thus, one way to control of chlorination is by monitoring the total chlorine residual. This process is known as Chlorine Control by Residual (CCR). In the CCR process, analytical measurements are made either manually (e.g., laboratory or field testing) or automatically (e.g., a process analyzer). All of the commonly used methods of analyses for CCR are based on classical iodometric chemistry. Iodide, added as a reagent, is oxidized by monochloramine, dichloramine and most organic chloramines to the tri-iodide ion: EQU NH.sub.2 Cl (monochloramine)-3I.sup.- +H.sub.2 O+H.sup.+.fwdarw.NH.sub.4 OH+Cl+I.sub.3.sup.- (tri-iodide ion) EQU NHCl.sub.2 (dichloramine)+3I.sup.- +H.sub.2 O+2H.sup.+.fwdarw.NH.sub.4 OH+2Cl.sup.- +I.sub.3 EQU OrgNH-Cl (organic chloramines)+3I.sup.-+H.sup.+.fwdarw.OrgNH.sub.2 +Cl.sup.- +I.sub.3 .sup.-
The resulting tri-iodide, which is formed in direct proportion to the amount of oxidant present, is measured in several ways:
1. Colorimetrically
A reagent indicator, such as N,N diethyl-p-phenylenediamine (DPD) is added and the tri-iodide oxidizes the indicator to a colored form, which can be measured by visual comparison, or suitable instrumentation (e.g., photometer, calorimeter or spectrophotometer). A variation of this technique is colorimetric titration, in which after reaction of the tri-iodide with DPD, the colored product is titrated against a suitable redox titrant, such as ferrous ammonium sulfate, to a colorless end-point.
2. Amperometrically
The tri-iodide ion is sensed by a suitable amperometric system, consisting of a probe or cell containing dual platinum electrodes or two dissimilar electrodes (e.g., silver/platinum) and a voltage generator. A small voltage is applied across the electrodes and the resulting current is compared to a standard reference potential. A variation of this technique is amperometric titration, in which the generated tri-iodide is reacted with a standard reducing titrant, such as phenylarsine oxide or sodium thiosulfate. The current will decrease with decreasing concentration of tri-iodide until no tri-iodide remains. The end-point is signaled when the current does not change. Another variation is known as the back-titration method, in which the released tri-iodide is reacted with an accurate excess amount of standard reductant, such as phenylarsine oxide or sodium thiosulfate. Then, the remaining reductant is titrated with standard iodate-iodide reagent. The end-point can be determined amperometrically or visually using the starch-iodide end-point.
3. Direct Titration with Visual Indication
The generated tri-iodide is titrated against standard thiosulfate titrant to a visual starch-iodide end-point.
The iodometric methods currently used for CCR are not specific for the preferred disinfectant, monochloramine. The CCR-iodometric process will tend to overestimate the disinfection efficiency due to the presence of the poorer disinfecting organic chloramines. Organic chloramines will be present in chlorinated wastewater due to poor mixing, chlorine transfer, or nitrification (which is explained below). Organic chloramines will interfere in all of the common residual analysis methods used for CCR. At the present time, there is no method for CCR based on maintaining a residual specific to the primary preferred disinfectant, monochloramine.
Under certain circumstances, secondary-treated wastewater may nitrify. During nitrification, the ammonia in the wastewater is partially oxidized to nitrite. With low ammonia levels, chlorination of nitrified waters will result in direct chlorination of any organic amines present. Thus, during a nitrifying event, the monochloramine disinfectant level in the chlorinated water may decrease and the organic chloramine level may increase. If nitrification occurs, conventional CCR processes may indicate an adequate disinfection level, when, in fact, disinfection efficiency has diminished.
A second process of controlling chlorination is by use of Oxidation-Reduction Potential (ORP). ORP is based on the concept that it is the oxidative potential derived from the residual that kills the microorganisms and not the concentration of the residual. Instead of maintaining a residual, ORP chlorination control maintains a certain ORP value, measured in millivolts. FIG. 2 shows typical ORP values for different concentrations of monochloramine, dichloramine and a mixture of three organic chloramines. The organic chloramine mixture tested included N-chloro-butylamine, N-chloro-diethylamine and a chlorinated tri-peptide of alanine. This mixture would be representative of organic chloramines found in chlorinated wastewater effluents.
As shown in FIG. 2, ORP can be used to distinguish between pure solutions of dichloramine and monochloramine, but cannot distinguish between monochloramine and any organic chloramines present. Hence, similar to CCR, the weaker disinfecting organic chloramines will also be indicated in ORP chlorination control.
Some wastewater facilities using chlorination find that they have difficulty in meeting their microbial limitations even though residual testing indicates the disinfectant concentration should be sufficient. Likewise, facilities that depend on ORP for chlorination control may experience difficulty in meeting effluent limits for disinfection, although ORP values indicate sufficient oxidation potential. In these cases, both CCR and ORP processes are indicating organic chloramines, which are weak disinfectants. This invention allows accurate chlorination control for the primary disinfectant residual without indicating organic chloramines.
There has not heretofore been described a process for controlling chlorination of wastewater or chloramination of drinking water having the features and advantages provided by the present invention.